1. Field of the Disclosure
The present disclosure relates to a method and apparatus for processing a transmission signal using a window function that changes a spectrum characteristic of a symbol, such as a spectrum emission mask or side-lobe attenuation in a spectrum.
2. Description of the Related Art
Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA) is a scheme that divides a data stream having a high transmission rate into multiple data streams with a low transmission rate, and transmits the multiple data streams in parallel using a plurality of subcarriers. OFDM/OFDMA has a high data transmission rate and high frequency efficiency, and may be robust to a frequency fading channel.
In Long Term Evolution (LTE) that is standardized by the 3rd Generation Partnership Project (3GPP), a Single Carrier-Frequency Division Multiple Access (SC-FDMA) scheme is used for uplink data transmission.
SC-FDMA is considered to be a modified version of OFDMA. In SC-FDMA, a transmitter transmits an information symbol using different orthogonal frequencies (subcarriers), as in OFDMA.
In OFDM/OFDMA/SC-FDMA/or SC-FDC (Single carrier frequency domain equalizer), Inter-Symbol Interference (ISI) may be removed by adding a guard interval, which is longer than a delay spread of a channel, between OFDM/OFDMA/SC-FDMA symbols (hereinafter, symbols) so as not to lose orthogonality of subcarriers due to a channel. A Cyclic Prefix (CP) or Cyclic Postfix is inserted into the guard interval to secure connectivity of the entire symbol section including the guard interval. That is, when a part of a symbol is copied and is inserted into a guard interval as a CP, and is disposed at the beginning of the symbol, the symbol is cyclically extended, thereby eliminating Inter-Symbol Interference (ISI) caused by multipath wireless channels.
OFDM/OFDMA/SC-FDMA may also implement parallel transmission of subcarriers using Inverse Fast Fourier Transform (IFFT) on a transmission side and Fast Fourier Transform (FFT) on a reception side. Accordingly, each of the subcarriers of an OFDM/OFDMA/SC-FDMA signal is formed in a sinc (=sin(x)/x) function, maintains orthogonality, and overlaps one another. Due to the characteristics of the sinc function, the OFDM/OFDMA/SC-FDMA signal is not band-limited and causes interference in an adjacent band.
To reduce adjacent channel interference, guard bands are reserved in band edges, in which no signal is transmitted through a few carriers in both ends of a corresponding band, instead of transmitting data through all subcarriers in an IFFT frequency band. However, a side-lobe of the sinc function itself is relatively large. To reduce adjacent channel interference only by increasing the length of guard band, the number of subcarriers through which data is not transmitted needs to be increased, which significantly decreases spectral efficiency.
Accordingly, time windowing is widely used as a method of reducing adjacent channel interference while maintaining spectral efficiency. The use of time windowing efficiently attenuates sidelobes in a spectrum caused by the sinc shape spectral leakage by applying gradually ascending or descending windowing to a Cyclic Prefix (CP) or both ends of data of a symbol, such that a rectangular shape of the symbol is smoothly increased or decreased. The side-lobe attenuation is essentially used for satisfying a spectrum emission mask or an Adjacent Channel Leakage Ratio (ACLR) specified in communication standards, which regulates the amount of interference to adjacent channels. A raised cosine window is mostly used among various windows used for the windowing scheme. For example, adjacent channel interference may be reduced by suppressing the amount of the spectral leakage outside a designated band using a window such as the raised cosine window.
A technique for attenuating the side-lobe spectrum of a signal may also use a lowpass/bandpass filtering scheme.
In the case of a coefficient value of the raised cosine window, however, a side-lobe attenuation rate in an output spectrum is determined, and an attenuation level is adjusted based on a roll-off factor associated with a change in a window length. As such, the raised cosine window occasionally fails to satisfy a spectrum emission mask. In this instance, a Low Pass Filter (LPF) including a large number of taps may be additionally used.
However, the spectral portion closest to the main-lobe of the spectrum requires digital processing since this portion is difficult to attenuate using an analog filter. Also, although standards requirements can be satisfied in a digital processing output, a side-lobe of an output signal may increase again due to a non-linear property of an RFIC or a Power Amplifier (PA), called spectrum regrowth. Therefore, it is desirable to provide enough margin in the spectrum emission mask or an ACLR by taking into account of the nonlinearity of the RFIC or PA together.